How can I make Christmas lights to put on a Christmas tree
You could do this with Modifiers, E.g. an Array modifier and a Curve modifier:


Create a stretch of wire with a light or two. The ends should be precisely placed so that the array modifier can merge them.


To model such a basic lowpoly cable, you could add a Circle with 5 vertices:



Then:
In edit mode, rotate 90 on the X axis (RX90), then extrude (E) it to both sides with increment snapping (Ctrl) so it is aligned to the grid:


Create two loop cuts (CtrlR) on either side of the middle edgeloop, then delete the middle edgeloop (Delete> Edge loop)


Scale the two top vertices apart (SY):


Extrude the two top faces up (E) and flatten by scaling to 0 (SZ0):


Extrude and scale in, extrude and scale up in a bulb shape, then merge (AltM) the remaining vertices:



add an Array modifier and enable Merge:


Add a Curve modifier and set the Object to a curve object:






Also see this tutorial on Blender Cookie